A battery system for an EV for example has a requirement to provide high performance under acceleration, start under load in initially cold or hot conditions, provide a long endurance service life, provide a high energy density to minimise overall weight, and survive a suitable warranty period without defect, as well as being as affordably priced as possible. EV battery packs currently in the market consist of multiple cells of a chosen cell chemistry, with a performance tailored as the best compromise of all the above characteristics.
Typically each type of cell chemistry has a particular internal impedance or electrical series resistance (ESR) that varies according to the rate of charge/discharge, and the temperature or age of the cells, and these generally differ from cell chemistry to cell chemistry.
A single cell chemistry cannot be tailored to be excellent in every respect, and a cell manufacturer working to improve cell performances in terms of range, lifespan and cost, may compromise on say charging efficiency. To help mitigate the drawbacks of individual cell chemistries, US 2013/0141045 discloses a battery pack with a main battery having a chemistry suited for a high power delivery, and a supplemental battery having a chemistry providing high energy density storage. The main and supplementary batteries are switched between and the main battery is used in preference to the supplementary battery.
Although, switching to the main battery in preference to the supplementary battery can result in the main battery becoming too depleted, requiring the supplementary battery to be used in place of the main battery and temporarily losing the benefit of the main battery until the battery pack can be recharged. For example, once the main battery becomes depleted, the ability of the vehicle to accelerate quickly may be significantly affected, even when a large charge still remains in the supplementary battery.
It is an aim of the invention to provide a more effective battery system for a comprehensive range of battery operating conditions, for example to provide a battery system that can work well in a combination of hot or cold conditions, under high power demands, that can have a high energy density, that can provide high cycle life, that can allow for partial battery replacements for lifed and/or defective cells, and optimise battery performance against cell cost. The term “lifed” as used herein refers to a cell which has significantly degraded in its performance over time, and so requires replacement.